There has been a conventional loose tube cable formed by packaging optical fibers in loose tubes to form loose tube-type optical fiber units and twisting these loose tube-type optical fiber units around a tension member, such as in Japanese Unexamined Patent Application Publication No. 2013-54219 (JP-A-2013-54219), for example.
When a loose tube includes single-core optical fibers inside, there may be differences between the lengths of the optical fibers, which are generated by uneven tensile force of the core wires or uneven lengths of the circumferences created at the time of manufacture, and this may deteriorate fiber skew (delay skew of light). In high-speed communication in particular, deterioration of fiber skew becomes a problem.
On the other hand, there is a method in which optical fiber ribbons are used. The optical fiber ribbon is an optical fiber ribbon that includes a plurality of optical fiber strands which are arranged in parallel and integrated. Using optical fiber ribbons allows the optical fiber strands forming the optical fiber ribbons to have the same length.
A loose tube is usually designed to have a large clearance between optical fiber ribbons and an inner surface of the loose tube therein. This is because, if this clearance is small and there is a change in temperature or a bending is given to the fiber, optical fiber strands positioned at both ends of the optical fiber ribbons are pushed toward inner walls of the loose tube, increasing the transmission loss.
As above, since it is necessary to provide a large clearance between the optical fiber ribbons and the inner surface of the loose tube, the diameter of the loose tube is required to be large. Thus, the loose tube itself becomes large.